Effect of the cation size disorder on charge ordering in rare-earth manganates

The effect of disorder arising from the mismatch between the average sizes of the A-site cations in the rare-earth manganates, ${L}_{0.5}{A}_{0.5}{\mathrm{MnO}}_{3}$ $(L=\mathrm{rare}\mathrm{}\mathrm{earth},$ $A=\mathrm{alkaline}\mathrm{}\mathrm{earth})$, on their charge-ordering behavior has been investigated. Measurements on two series of manganates with a fixed average cation radius, $〈{r}_{A}〉,$ of 1.17 and 1.24 \AA{}, reveal that the charge-ordering transition temperature, ${T}_{\mathrm{CO}},$ is not very sensitive to the size mismatch unlike ferromagnetic ${T}_{C}$ or metal-insulator transition data. Furthermore, ${T}_{\mathrm{CO}}$ is not affected greatly by the orthorhombic lattice distortion. The observed decrease in ${T}_{\mathrm{CO}}$ with increase in $〈{r}_{A}〉$ is consistent with a quadratic dependence upon ${(r}_{A}^{0}\ensuremath{-}〈{r}_{A}〉)$ where ${r}_{A}^{0}=1.13\AA{}.$